The present invention relates to a photodetector sensitive to infrared radiation. In particular, the present invention provides for a grating coupled photodetector sensitive to radiation in two different infrared wavelength bands and that provides two corresponding output signals.
In the field of infrared (IR) imaging, the current objective is to provide large area focal plane arrays at low cost with high performance. InSb, HgCdTe, and quantum well infrared photodetector (QWIP) technologies have demonstrated high performance large area focal plane arrays. Each of these technologies has various strengths and weaknesses. InSb photodetectors offer high performance and ease of fabrication, but must be cooled to approximately 80 K. HgCdTe photodetectors can be designed to operate in either the long wavelength IR (LWIR) corresponding to a wavelength range of 8 to 12 xcexcm or the middle wavelength IR (MWIR) corresponding to a wavelength range of 3 to 5 xcexcm. However, HgCdTe photodetectors require very tight tolerances in material and fabrication uniformity to ensure high performance. QWIP photodetectors have been demonstrated in both the MWIR and LWIR requiring only moderate tolerances in both material and fabrication uniformity. However, QWIP photodetectors"" performance is generally lower than that achieved by InSb or HgCdTe photodetectors.
Two-color detection is increasingly desirable as a method to increase the probability of detection under various environments. As an example, objects that are only slightly above room temperature, such as a person, are most easily detected in the LWIR corresponding to the peak IR radiation emission band for room temperature objects. In contrast, a hot object, such as an automobile exhaust pipe, can be readily detected in the MWIR corresponding to the peak IR radiation emission band for objects having a temperature more than 600 K. Thus, a system that must provide high performance with either of these objects must be sensitive to both wavelength bands.
In military applications, it is possible to camouflage an object such that the object emits little radiation in a particular portion of the IR spectrum. A two-color photodetector with appropriately selected sensing wavelengths therefore provides a means of detecting objects that have been camouflaged in this manner.
Additional applications may use two-color photodetectors for discriminating one object from another. As two objects at different temperatures emit different amounts of IR radiation at different wavelengths, a two-color photodetector can be used to more readily discriminate between the objects. As an example, two identical cars may be parked next to each other. If one has recently been driven while the second has not been operated for several hours, a two-color detector could readily sense the difference in exhaust pipe or tire temperatures.
However, conventional IR detector technologies have proven difficult to adapt to this current demand for two-color detection. As noted above, high performance single color detectors and imaging arrays have been demonstrated using HgCdTe, InSb, and QWIP technologies. Of these, two-color detection is possible only with the HgCdTe and QWIP technologies. The two-color HgCdTe photodetectors demonstrated to date have suffered significantly from both non-uniformity in the HgCdTe material and the fabrication process. While two-color QWIP photodetectors do not place as stringent requirements upon the starting material, the fabrication process has similarly proven quite difficult. Further, both IR detector technologies have suffered from reduced performance in two-color photodetectors in comparison to single color performance.
In view of the desirability of two-color IR photodetectors, there exists a need for a design that places fewer and/or less stringent requirements upon the starting material and/or the fabrication process.
In one embodiment of the present invention, a two-color IR photodetector comprises in order, a top electrical contact, a top IR absorbing layer for absorbing a first color, a middle electrical contact, a bottom IR absorbing layer for absorbing a second color, a bottom electrical contact, and a reflector. The top contact and the top IR absorbing layer form a first diffractive grating for the first color. The middle contact and the bottom IR absorbing layer form a second diffractive grating for the second color. The first diffractive grating has a period that is an integer multiple of the period of the second diffractive grating.
In another embodiment of the present invention, a two-color IR photodetector comprises in order, a top electrical contact, a top IR absorbing layer for absorbing a first color, a middle electrical contact, a bottom IR absorbing layer for absorbing a second color, a bottom electrical contact, and a reflector. The top contact, the top IR absorbing layer, the middle contact, and the bottom IR absorbing layer form a doubly periodic diffractive grating. The doubly periodic diffractive grating has a period in a first direction to diffract the first color and a period in a perpendicular direction to diffract the second color.
In yet another embodiment of the present invention, a two-color IR photodetector comprises in order, a top electrical contact, a top IR absorbing layer for absorbing a first color, a middle electrical contact, a Bragg reflector, a bottom IR absorbing layer for absorbing a second color, a bottom electrical contact, and a reflector. The top contact and the top IR absorbing layer form a diffractive grating for the first color. The Bragg reflector, the bottom IR absorbing layer, the bottom contact, and the reflector form a vertical resonant optical cavity for the second color.
In each embodiment, a first bias is placed across the top IR absorbing layer via the top and middle electrical contacts such that a resulting first current flow is along an axis of the top IR absorbing layer. The magnitude of the resulting first current flow is indicative of the quantity of the first color of IR radiation absorbed by the top IR absorbing layer. Likewise, a second bias is placed across the bottom IR absorbing layer via the middle and bottom electrical contacts such that a resulting second current flow is along an axis of the bottom IR absorbing layer. The magnitude of the resulting second current flow is indicative of the quantity of the second color of IR radiation absorbed by the bottom IR absorbing layer.
Photodetectors comprising a single two-color detector, a one-dimensional line array of photodetectors, or a two-dimensional area array of photodetectors are envisioned. Depending upon the specific embodiment, a number of different material systems may be used to form the starting material.